Increased understanding of the role of carbohydrates as recognition elements on the surface of cells has led to increased interest in the production of carbohydrate molecules of defined structure. For instance, compounds comprising the oligosaccharide moiety, sialyl lactose, have been of interest as neutralizers for enterotoxins from bacteria such as Vibrio cholerae, Eschericia coli, and Salmonella (see, e.g., U.S. Pat. No. 5,330,975). Sialyl lactose has also been investigated for the treatment of arthritis and related autoimmune diseases. In particular, sialyl lactose is thought to inhibit or disrupt the degree of occupancy of the Fc carbohydrate binding site on IgG, and thus prevent the formation of immune complexes (see, U.S. Pat. No. 5,164,374). Recently, sialyl.alpha.(2,3)galactosides, sialyl lactose and sialyl lactosamine have been proposed for the treatment of ulcers, and Phase I clinical trials have begun for the use of the former compound in this capacity. See, Balkonen, et al, FEMS Immunology and Medical Microbiology 7:29 (1993) and BioWorld Today, p. 5, Apr. 4, 1995. In addition, sialyl lactose is useful as food supplement, for instance in baby formula.
Because of interest in making desired carbohydrate structures, glycosyltransferases and their role in enzyme-catalyzed synthesis of carbohydrates are presently being extensively studied. These enzymes exhibit high specificity and are useful in forming carbohydrate structures of defined sequence. Consequently, glycosyltransferases are increasingly used as enzymatic catalysts in synthesis of a number of carbohydrates used for therapeutic and other purposes.
In the application of enzymes to the field of synthetic carbohydrate chemistry, the use of sialyltransferases for enzymatic sialylation offers advantages over chemical methods due to the virtually complete stereoselectivity and linkage specificity offered by the enzymes (Ito et al., Pure Appl. Chem., 65:753 (1993) U.S. Pat. Nos. 5,352,670, and 5,374,541).
Improved methods for enzymatic synthesis of sialylated carbohydrate compounds would advance the production of a number of beneficial compounds. The present invention fulfills these and other needs.